The invention relates to a drive assembly for motor vehicles with front wheels and rear wheels, especially for tractors, agricultural machinery, construction machinery or self-propelled machines.
The prior art illustrate fully hydro-statically driven vehicles which use a hydraulic pump to be driven by an engine. The hydraulic pump, in turn, drives hydraulic motors at the driving axles of the wheels. In order to continuously vary the driving speed, the hydraulic pump is in the form of an adjustable pump and/or the hydraulic motors are in the form of adjustable motors. However, as a rule, the range of adjustment of the hydraulic pump and of the hydraulic motors is normally only sufficient to cover a driving speed range up to approximately 25 km/h. In order to enable higher driving speeds, prior art drive concepts are usually provided with an unsynchronised two-stage gearbox. Thus, it is possible for the driver to choose between two gears. One gear is for lower speeds and one gear is for higher speeds. However, because of the normally unsynchronised nature of the gearbox, the vehicle has to be stopped before the gear-changing operation can take place.
As a rule, hydrostatic drive systems include one driving speed range where the coefficient of efficiency is clearly higher than in the remaining driving speed ranges. Normally, a hydrostatic drive system is designed such that the high coefficient of efficiency is reached at mean driving speeds. However, when the vehicle is driven at an extremely low speed or at higher speeds, the coefficient of efficiency is unacceptably low.
Furthermore, prior art four-wheel drives with a hydrostatic drive have a rigid connection between the front axle and the rear axle. In curves, the rigid connection is disadvantageous because the curve radius of the front wheels is greater than the curve radius of the rear wheels. This results in torsion in the drive elements and in the driveshafts. The torsion occurring in the drive elements results in reactive power which, depending on the wheel contact area, leads to slip at the front and rear wheels. The slip, in turn, leads to greater tire wear or, if the vehicle is driven in fields, it leads to the turf being torn up.